The present invention relates to Raman spectroscopy and surface-enhanced Raman spectroscopy; and more particularly to surface-enhanced Raman medical (SERMED) diagnostic instruments and methods for non-invasive medical diagnosis and drug screening.
Normal Raman spectroscopy relates to the scattering of light by a gas, liquid or solid with a shift in frequency or wavelength from that of the usually monochromatic incident radiation. Upon irradiation of a molecule with light in biological applications, the incident radiation having a frequency .nu. should produce scattered radiation, the most intense part of which has unchanged frequency (Rayleigh scattering). In addition, if the polarization of a molecule changes as it rotates or vibrates, there are spectral lines of much lesser intensity at frequencies .nu..+-..nu..sub.k, where .nu..sub.k is the molecular frequencies of rotation or vibration.
Fleischmann et al. first reported strongly enhanced Raman scattering from pyridine molecules adsorbed on silver electrode surfaces that had been roughened electrochemically by oxidation-reduction cycles (Chem. Phys. Lett. 26, 163, 1974). This increase in Raman signal, originally attributed to a high surface density produced by the roughening of the surface of electrodes, was later identified by Jeanmaire and Van Duyne (J. Electroanal. Chem. 84, 1, 1977) and independently by Albrecht and Creighton (J. Am. Chem. Soc. 99, 5215, 1977) as a direct result of a surface-enhancement process, hence the term surface-enhanced Raman scattering (SERS) effect.
There are at least two major types of mechanisms that contribute to the SERS effect: a) an electromagnetic effect associated with large local fields caused by electromagnetic resonances occurring near metal surface structures, and b) a chemical effect involving a scattering process associated with chemical interactions between the molecule and the metal surface. It has been shown that electromagnetic interactions between the molecule and the substrate provide one of the dominant enhancements in the SERS process. Such electromagnetic interactions are divided into two major classes; interactions that occur only in the presence of a radiation field, and interactions that occur even without a radiation field. The first class of interactions between the molecule and the substrate are believed to play a major role in the SERS process. A major contribution to electromagnetic enhancement is due to surface plasmons. Surface plasmons are associated with collective excitations of surface conduction electrons in metal particles. Raman enhancements result from excitation of these surface plasmons by the incident radiation. At the plasmon frequency, the metal becomes highly polarizable, resulting in large field-induced polarizations and thus large local fields on the surface. These local fields increase the Raman emission intensity, which is proportional to the square of the applied field at the molecule. Additional enhancement is due to excitation of surface plasmons by the Raman emission radiation of the molecule.
Surface plasmons are not the only sources of enhanced local electromagnetic fields. Other types of electromagnetic enhancement mechanisms are concentration of electromagnetic field lines near high-curvature points on the surface, i.e., the "lightning rod" effect, polarization of the surface by dipole-induced fields in absorbed molecules, i.e., the image effect, and Fresnel reflection effects.
The chemical effect is associated with the overlap of metal and adsorbate electronic wave functions, which leads to ground-state and light-induced charge-transfer processes. In the charge-transfer model, an electron of the metal, excited by the incident photon, tunnels into a charge-transfer excited state of the adsorbed molecule. The resulting negative ion (adsorbate molecule-electron) has a different equilibrium geometry than the original neutral adsorbate molecule. Therefore, the charge-transfer process induces a nuclear relaxation in the adsorbate molecule which, after the return of the electron to the metal, leads to a vibrationally excited neutral molecule and to emission of a Raman-shifted photon. The "adatom model" also suggests additional Ramon enhancement for adsorbates at special active sites of atomic-scale roughness, which may facilitate charge-transfer enhancement mechanisms.